Experiments are being carried out to determine the neural systems and neuronal mechanisms of habituation and sensitization of an elementary response to loud acoustic stimuli, the acoustic startle response in rats. Single neuron recording during iterated sensory stimulation will be undertaken in locally anesthetized, paralyzed rats. Activity to repetitive sensory stimulatton, and plasticity of such activity, will be characterized as it occurs in the reticular formation and hippocampal formation. Comparisons between plasticity of neuronal activity and plasticity of the acoustic startle response will be undertaken, including both short-term and long-term decrements in neuronal activity and behavior. Lesion experiments will be used to determine the nature of participation of different sensory and non- specific pathways in the brainstem and forebrain in the acoustic sartle response and the elementary forms of behavioral plasticity that it shows.